Colonies of animals with congenital coagulation, platelet function, and complement defects are maintained for breeding purposes, genetic and physiologic studies, production of reagents for coagulation tests and evaluation of hemostasis. These include dogs with hemophilia A, hemophilia B, hemophilia AB, von Willebrand's disease, factor VII deficiency, factor X deficiency, factor XI deficiency, dysprothrombinemia, thrombasthenic thrombopathia, and various combinations of these defects; fawn-hooded rats with platelet serotonin deficiency (storage pool disease); Flemish Giant-Chinchilla rabbits with a von Willebrand's-like autosomal factor VIII deficiency; C4-deficient guinea pigs; and Syrian hamsters with an age-associated spontaneous atrial thrombosis. Studies with these spontaneous animal models examine basic mechanisms of hemostasis and thrombosis in comparison to analogous human diseases. Special emphasis is on the biologic, immunologic, and physiologic properties of canine and rabbit factor VIII. This includes studies of plasma, tissue, and platelet factor VIII coagulant activity, factor VIII-related antigen, and von Willebrand's factor activities (bleeding time, platelet retention, and ristocetin-induced platelet aggregation).